Tazzle is used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH). Part of the physiological process of erection involves the release of nitric oxide (NO) in the corpus cavernosum. This then activates the enzyme guanylate cyclase, which increases cyclic guanosine monophosphate (cGMP) levels, leading to smooth muscle relaxation in the corpus cavernosum, resulting in an increased inflow of blood and an erection. Tazzle is a potent and selective inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5) responsible for cGMP degradation in the corpus cavernosum. This means that, with tadalafil on board, normal sexual stimulation leads to increased levels of cGMP in the corpus cavernosum which leads to better erections. Without sexual stimulation and no activation of the NO/cGMP system, tadalafil should not cause an erection.